1. Field of the Invention
This invention relates to an electromagnetic deflection assembly for a cathode ray tube.
2. Description of the Prior Art
In a color cathode ray tube, three separate electron beams (blue, green and red) are caused to scan across the faceplate of the cathode ray tube. Each beam is arranged so that is illuminates only phosphor dots corresponding to its color. Thus the blue beam should only strike blue phosphor dots, the green beam should only strike green phosphor dots, and the red beam should only strike red phosphor dots. To this end, color cathode ray tubes, in addition to the scanning yokes which will be found on cathode ray tubes, are provided with controls to adjust the positions of the three beams relative to one another. Commonly, controls are provided which correct for convergence of the three beams and for the purity of the resulting colors.
British Pat. No. 1,428,678 discloses a so-called lateral-blue convergence assembly in which a strip of high permeability magnetic material is bent to form six pole pieces intended to be spaced around the neck of the cathode ray tube. Coils are provided around three of the pole pieces and by suitable positioning the pole pieces and selecting appropriate currents, the blue beam can be shifted laterally of itself.
Purity rings consisting of a pair of magnetized annular magnets have been used in the past for adjusting the purity. The two magnets are rotatable about the neck of the cathode ray tube and relative to one another so as to produce a magnetic field within the neck whose magnitude and direction can be varied by rotation of the magnets. More recently, the use of electromagnetic coils instead of permanent magnets has been suggested for purity adjustment.
Until now, most color cathode ray tubes have been produced for domestic television sets which typically may have 625 scan lines on the screen. Such cathode ray tubes are not very suitable for displaying text or graphic images because of their limited resolution. The cathode ray tube industry is now beginning to produce high resolution color monitors which are designed to display text and graphical data to a much higher resolution than normal domestic television sets. Such high resolution monitors are characterized by a larger number of phosphor dots, a larger number of holes in the shadow mask tube, and much more stringent requirements as to the accuracy of the scanning coils etc. Pin cushion distortion and other similar defects are much more visible when displaying text and graphical data than when displaying moving or still pictures. It is important that these stiffer requirements do not impose too high a cost penalty.